A number of catalysts have been proposed for the production of acrylonitrile by the ammoxidation of propylene. Particularly, various multi-component molybdenum/bismuth catalysts are used as these catalysts. Examples of these processes include those disclosed by JP-B-36-5870 (U.S. Pat. No. 2,904,580) (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-38-l7967 (U.S. Pat. No. 3,226,422), JP-B-39-85l2, JP-B-45-35287, JP-A-48-47476 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-5l-33888 and JP-B-60-368l2 (U.S. Pat. Nos. 4,162,234 and 4,377,534). Thus, improvements in these processes have been attempted from various viewpoints.
However, the use of these catalysts in a fluidized bed reaction is accompanied by problems in that the resulting acrylonitrile yield is not always satisfactorily high; that it is difficult to prepare a catalyst with sufficient strength; and that certain expensive materials such as cobalt should be used as a catalytic component. Accordingly, improvement in these known processes from an industrial viewpoint has been desired.
Some processes regarding the preparation of a molybdenum/bismuth fluidized bed catalyst are known. For example, JP-B-37-8568 (U.S. Pat. No. 3,044,965) teaches a process comprising spray-drying an ammonium nitrate-containing slurry comprising molybdenum, bismuth and silica. JP-B-54-l29l3 discloses a process for the preparation of a molybdenum/bismuth/iron/sodium/phosphorus catalyst of a highly restricted composition range. JP-B-57-49253 (U.S. Pat. No. 3,746,657) discloses a process wherein a silica sol is added stepwise to an aqueous suspension containing a heptamolybdate at a temperature lower than 66.degree. C. and then phosphoric acid and other components such as bismuth, iron, cobalt and nickel are further added thereto. Further, JP-A-55-56839 (U.S. Pat. No. 4,264,476), JP-A-55-l39839 (U.S. Pat. No. 4,290,922), JP-A-57-65329 and JP-A-57-75l47 each disclose a process for the preparation of a molybdenum/bismuth hardwearing fluidized catalyst wherein the pH value and temperature of a slurry containing the catalytic components are adjusted to 1 or below and 60.degree. C. or below, respectively. JP-A-55-l3l87 (U.S. Pat. No. 4,212,766) discloses a process comprising preliminarily forming molybdate(s) and/or tungstate(s) of Bi, Te, Sb, Sn, Cu or a mixture thereof in an aqueous slurry prior to combining the same with other elements. Furthermore, JP-A-57-l2827 discloses a process wherein a slurry containing molybdenum/bismuth/ antimony is heated to 40.degree. to 150.degree. C. for at least 30 minutes.
As described in JP-B-48-43096 (U.S. Pat. No. 3,766,092), however, a molybdenum/bismuth catalyst which does not contain cobalt gives only a low acrylonitrile yield. Further, it is difficult to obtain a fluidized bed catalyst having a strength suitable for a fluidized bed reaction in this case.